Received an automated bill after Xfinity service was cancelled, now a collection agency has asked for the money.

I cancelled my Xifnity service when I moved out of an apartment in April. I talked to a service rep on the phone to cancel service on April 15 (in an excruciating 1 hour phone call where they wouldn't let up on offering me special deals again and again. I simply wanted to cancel service). I had left the apartment on April 10 and was no longer using the service. I received an email from Xfinity on April 16 that said "We received your cancellation request."

My last auto payment had been on March 20, so I would think maybe I was entitled to 4 or 5 days worth of refund. 

I received an email on May 24 saying my Xfinity payment of $93.60 is now due. As I had cancelled my service, I ignored this. 
Today July 7, I received a letter and a text message from a debt collection agency asking for that money. 

The equipment was returned. The $93.60, according to the debt collector, is not for the equipment but for service. Why? I haven't been using the service. Can you cancel this debt and call them off?